On September 11, 2001, terrorists crashed two hijacked commercial jets into the Twin Towers of New York City's World Trade Center in a coordinated attack on this symbol of American financial power and influence. Remarkably, both towers survived the initial impact of Boeing 767 jets traveling at speeds in excess of 400 mph, and remained standing long enough for most occupants below the impact floors to escape.
In each tower, 236 perimeter columns formed a highly redundant structural steel tube that redistributed loads around severed columns, preventing immediate collapse. The force of the impacts atomized the jet fuel in both aircraft and also stripped fireproofing from the intact steel elements of the towers. Approximately a third of the jet fuel was consumed in the highly visible fireballs at the points of impact. The rest of the fuel was forced into the towers and saturated the combustible contents. The fires caused by the ignition of these materials directly heated the exposed and overloaded columns and trusses, rapidly causing the steel to lose strength. After 56 minutes for the south tower and 102 minutes for the north tower, the strength of critical members fell below the loads they were carrying, initiating collapse. Thousands of tons of structural debris destroyed the surrounding and below grade portions of the World Trade Center complex, severed vital underground utility and communications links, and damaged numerous surrounding buildings. The smoke and dust plumes from the collapse were visible from outer space.
In the months following the disaster, Exponent was retained by the insurers of the WTC to provide a detailed analysis of the effects of the attack on the Twin Towers and surrounding buildings. Specifically, Exponent conducted a detailed study of the collapse and debris patterns for each tower and analyzed damage to the sub-grade mechanical, electrical and plumbing infrastructure to predict the effects of heavy and light debris impacts caused by the collapse of each tower. Our analysis included the effects of fire spread, smoke, and dust contamination and wind tunnel studies of the performance of the towers.
Colwell J, Mongia R, Reza A. Case study on evacuation rates within the World Trade Center towers on September 11, 2001. Proceedings, 49th Annual Human Factors and Ergonomics Society Meeting, Orlando, FL, September 26–30, 2005; also in Proceedings, DRI Fire and Casualty Seminar, November 17–18, 2005.
Reza A. Picture this: Using hypothetical loss scenarios for forensic damage and claims evaluations—A case study based on the 9/11 attack on the World Trade Center. The ARC Group Canada Launch Event, Toronto, October 26, 2006.